This invention relates in general to a pulser for generating a pulse by switching the state of a magnetic device that has come to be known as a Wiegand wire and, more particularly, to a mechanism and method for controlling the magnetic field to which a Wiegand wire module is subjected so as to provide an improved output pulse.
The magnetic device employing the pulser of this invention is of the type disclosed in U.S. Pat. No. 4,247,601 issued Jan. 27, 1981. This magnetic device is a ferro-magnetic wire segment which has been treated to provide core and shell portions with divergent magnetic properties. The wire is now known in the art as a Wiegand wire.
The Wiegand wire essentially has two states. In one of these states, the magnetization of the core and shell are in opposite directions and this state may conveniently be called a reverse state. In the other state, the magnetization of the core and shell are in the same direction and this state may conveniently be called the confluent state. When the magnetic field to which the wire is subjected passes a threshold in one direction or the other, the wire switches state. The switch in state is extremely rapid so that the rate of change of flux through a pick-up coil wrapped around the wire is great. As a consequence the output from the pick-up coil is very substantial, in some cases being as high as eight volts into an open circuit on a repeatable basis. The wire and pick-up coil is referred to as a module.
The characteristics of a pulse generator which are desirable include simplicity, low cost, versatility, repeatability, reliability and high output pulse. There are inevitably certain trade-offs in enhancing certain of these characteristics at the expense of other characteristics. However, it is desirable to provide an optimum combination of these characteristics with whatever enhancement of the characteristics may be achieved in an improved design and such is the general purpose of this invention.
U.S. Pat. No. 4,309,628 issued Jan. 5, 1982, discloses an arrangement in which a Wiegand wire module is triggered by a shunt element. Disclosed therein are various shunt structures which might be employed to provide a shunt trigger for the Wiegand wire. Disclosed in that patent are (1) a non-ferrous disk with low reluctance elements deployed on the face of the disk, (2) a rotating non-ferrous drum with low-reluctance elements deployed around the rim of the drum, (3) a ferrous rotating disk with a series of ferrous lobes that protrude radially from the rim of the disk, and (4) a ferrous rotating disk with a plurality of cut-out zones on the face of the disk.
U.S. Pat. No. 4,484,090 entitled Pulsed Generator With Shaped Magnetic Field issued Nov. 20, 1984 discusses the problem that arises because of the demagnetization field at the ends of the Wiegand wire. That patent discloses a pulser in which the excitation field is shaped so as to minimize the axial field at the ends of the wire thereby minimizing the effects of the demagnetization field at the end of the wire. This shaped field design provides an improved pulser in that the more rapid switch in flux when the demagnetization field is minimized provides a larger output pulse in the pickup coil than is the case when the field at the ends of the wire is significant. The increased pulse magnitude provided by the shaped field design disclosed in this patent serves to enhance the utility of the pulser. However, it has been found that the improvement provided is not as consistent and repeatable as is desirable in operating equipment. In particular, the effects of ambient fields and, most particularly, changing ambient fields results in a loss of repeatability and consistent pulse output magnitude.
Accordingly, it is a purpose of this invention to provide a pulser in which the enhanced output pulse obtained by a shaped magnetic field is provided in a repeatable and consistent fashion.
It is a related purpose of this invention to provide such a pulser as it will be fairly insensitive to the environmental conditions, will be relatively easy to calibrate and will be relatively tolerant of variations in the spacing of the excitation shunt so as to provide a consistent pulse output and, in particular, to provide a symmetric output in which the alternate positive going and negative going pulses are substantially similar in absolute magnitude.
The co-pending parent patent application discloses a pulser which achieves this purpose in a device actuated by a magnet passed over the pulser unit to magnetically couple to the Wiegand wire thereby switching the state of the Wiegand wire and generating an appropriate output pulse. However, there are applications for a pulse generating unit where it is desirable to generate the pulse by bringing a low reluctance ferro-magnetic shunt adjacent to the Wiegand wire module. Applications involving high pulse count preferably use a shunt trigger rather than a magnet trigger because it is less costly to employ multiple ferro-magnetic shunts rather than multiple high quality magnets. One application for such a shunt triggered pulser is in an anti-skid device where the pulser is used as part of a velocity sensor and may generate a hundred pulses during a revolution of the wheel of the vehicle.
Accordingly, it is an object of this invention to provide all of the above-referenced objects in the context of a shunt actuated pulser.